1. Field of the Invention
The present invention relates to wireless communications and, in particular, to a method and apparatus for transmitting/receiving control channels through a wireless backhaul in an OFDM-based communication system including relay nodes.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation technique in which a serial input symbol stream is converted into parallel symbol streams and modulated into mutually orthogonal subcarriers, i.e., a plurality of subcarrier channels.
The multicarrier modulation-based system was first applied to military high-frequency radios in the late 1950s, and the OFDM scheme, which overlaps multiple orthogonal subcarriers, has been developing since the 1970s. There were limitations on its application to actual systems due to the difficulty in realization of orthogonal modulation between multiple carriers. However, the OFDM scheme has undergone rapid development since Weinstein et al. presented in 1971 that OFDM-based modulation/demodulation can be efficiently processed using DFT (Discrete Fourier Transform). In addition, as a scheme that is known as one that uses a guard interval and inserts a Cyclic Prefix (CP) symbol into the guard interval, the negative influence of the system on the multiple paths and delay spread has been reduced significantly.
Owing to such technical developments, OFDM technology is being widely applied to digital transmission technologies such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Wireless Local Area Network (WLAN), Wireless Asynchronous Transfer Mode (WATM), etc. That is, the OFDM scheme could not be widely used before due to its high hardware complexity, but the development of various digital signal processing technologies including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) has enabled its realization.
OFDM, though it is similar to the conventional Frequency Division Multiplexing (FDM), can obtain optimal transmission efficiency during high-speed data transmission by maintaining orthogonality between multiple tones. In addition, the OFDM scheme can obtain optimal transmission efficiency during high-speed data transmission as it has high frequency utilization efficiency and is robust against multipath fading.
Since OFDM overlaps the frequency spectra of the subcarriers, it has high frequency utilization efficiency, is robust against frequency selective fading, can reduce an Inter-Symbol Interference (ISI) effect with the use of a guard interval, can facilitate design of the simple hardware of an equalizer, and is robust against impulse noises. Therefore, the OFDM scheme is used for various communication systems.
In wireless communications, high-speed, high-quality data services are generally hindered by channel environment. In wireless communications, channel environments suffer from frequent changes not only due to Additive White Gaussian Noise (AWGN) but also power variation of received signals, caused by a fading phenomenon, shadowing, a Doppler effect due to movement of a terminal and a frequent change in a velocity of the terminal, interference by other users or multipath signals, etc. Therefore, in order to support high-speed, high-quality data services in wireless communication, there is a need to efficiently overcome the above factors.
In OFDM, modulation signals are located in the two-dimensional time-frequency resources. Resources on the time domain are divided into different OFDM symbols, and are orthogonal with each other. Resources on the frequency domain are divided into different tones, and are also orthogonal with each other. That is, the OFDM scheme defines one minimum unit resource by designating a particular OFDM symbol on the time domain and a particular tone on the frequency domain, and the unit resource is called a Resource Element (RE). Since different REs are orthogonal with each other, signals transmitted on different REs can be received without causing interference to each other.
A physical channel is a channel defined on the physical layer for transmitting modulation symbols obtained by modulating one or more coded bit sequences. In an Orthogonal Frequency Division Multiple Access (OFDMA) system, a plurality of physical channels can be transmitted depending on the usage of an information sequence or receiver. The transmitter and receiver negotiate the RE on which a physical channel is transmitted, and this process is called mapping.